Processes for casting molten metal in active carbon coated ceramic shell moulds



United States Patent Office Patented Oct. 28, 1969 US. Cl. 164-23 ClaimsABSTRACT OF THE DISCLOSURE The process of the disclosure is one for theproduction of a metal casting, in which molten metal is poured into aceramic shell mould, and the casting and shell are allowed to cool whilesurrounded by active carbon in particulate form.

This invention relates to a process for the production of metalcastings, in particular to a process of metal casting employing ceramicshell moulds.

A problem which has for a long time hindered the manufacture ofsatisfactory castings from certain types of ferrous metals, for exampleplain carbon steels, low-alloy steels and many ferritic and martensiticstainless steels, stems from the fact that such metals are oxidized athigh temperatures. Under normal conditions of operation, the relativelyporous structure of a ceramic shell mould can permit oxidation of themetal adjacent to the shell wall to occur. The nature of the resultingchanges in the metal surface vary according to the conditions and theparticular metal concerned, but they can be such that the appearance andgeneral quality of the castings are adversely affected.

Various proposals have been made for dealing with this problem, forexample that the ceramic shell mould should be located in anon-oxidizing atmosphere during the pouring and cooling of the metal. Ithas also been proposed to surround the mould with carbon at asufliciently high temperature to combine with the oxygen in the vicinityof the mould, thus preventing its access to the metal. While the formsof carbon proposed hitherto, for example graphite and anthracite, ingeneral give reasonably satisfactory results in the casting of theferritic-martensitic stainless steels containing in the region of 13% byweight of chromium, they do not function satisfactorily with all alloysin this group, and are generally inadequate to prevent surfacedisfigurati'on in castings of plain lowcarbon steels, certain low-carbonlow-alloy steels, and high alloy but non-stainless tool steels such asthe German specification X165 Cr Mo V12 (DIN 17,006).

We have now found that certain other forms of carbonaceous material aresignificantly more effective, giving improved results throughout therange of stainless steels mentioned above, making possible theproduction of high quality castings from plain low-carbon steels,low-carbon low-alloy steels, and high alloy tool steels, and also makingpossible the use of thinner ceramic shells which hitherto would have hadunacceptably high porosity.

The casting and shell are allowed to cool under these conditions to atemperature below the minimum at which exposure to air would have anadverse effect. Whenever practicable, the shell mould should besurrounded by the carbon before the metal is poured, but if this cannotbe arranged, immersion in the carbon should be effected as soon aspossible, for example during casting or immediately on its completion.

Active carbons are carbons characterized by high porosity andcorrespondingly high surface areas. The production of active carbonsusually involves a first stage in which the raw material, for examplebone, wood, peat of nut shells, is carbonised, normally by heating inthe absence of air. In the second stage the resulting char is subjectedto an activation process. Several such processes have been proposed, andone which is used extensively involves controlled oxidation of thecarbon with suitable gases. For example steam or carbon dioxide can beused at temperatures of 800900 C., or air can be used at 300-600 C. Theoxidizing gases remove residual hydrocarbons and other volatile materialand cause an erosion of the carbon surface.

Active carbons generally have specific suface areas of at least squaremetres per gram, and for use in the present invention, active carbonshaving specific surface areas of at least 500 square metres per gram,and more especially in the range 1000-1600 square metres per gram arepreferred. '(The surafce area is usually determined by gas absorptiontechniques based on the procedure of Brunauer, Emmett and Teller). Afurther preferred feature is that the active carbon should have aresidual volatile content not exceeding 5% by weight. Active carbonsderived from charcoals of vegetable origin have given particularly goodresults in the present process, especially an active carbon produced bythe pyrolysis of coconut shells.

In respect of particle size, we prefer to use somewhat finer materialthan that indicated as suitable where other forms of carbon have beenproposed. Preferably, substantially all the particles should passthrough a 16 B.S.S. (British Standard Sieve) mesh, and more preferablysubstantially all should pass through a 30 B.S.S. mesh. At the lower endof the particle size range, material that contains any substantialproportion of particles passing a 200 B.S.S. mesh is rather dusty, andwhile effective for the process of the invention is inconvenient tohandle, and tends to generate dirty working conditions. It is thereforepreferred to use carbon substantially all of which is retained by B.S.S.mesh. Commercially, the active carbons are available in various gradescorresponding to different ranges of particles size; grades havingparticles size ranges of -30+80 and -52+l20 B.S.S. mesh have been usedvery successfully. A grade having a particle size range of 16+60 B.S.S.mesh has also been shown to be satisfactory.

Mixtures of active carbon and refractory solids in particulate form, forexample Molochite, zircon sand or metal ball-shot as used for shotblasting, can be used to surround the mould in the process of thepresent invention. In certain circumstances such mixtures are effectivewhen containing as little as 5% by weight of the active carbon, althoughthe minimum which it is possible to use in any. particular instance willdepend on a variety of factors including in particular thesusceptibility of the metal to oxidation.

By suitable choice of components, mixtures can be selected to givefaster rates of cooling than can be achieved using carbon alone. Thismay be desirable if thick sections of metal are being cast and if themetal is such that a slow rate of cooling is associated with thedevelopment of surface porosity.

While the process of the invention can be used for production ofcastings of high chromium steels, its particular advantages lies in thefact that it permits satisfactory casting of plain low-carbon steels,for example BS.1617A, BS3146 and CLA.9; of low alloy steels, for examplethose of the Fortiweld type and type EN36C; and high-carbon, high-alloy,tool steels generally containing around 12% Cr, and other alloys whichhave poor high-temperature scaling resistance.

It is contemplated that the ceramic shell mould for use in the processof the present invention will have been produced by conventional means.The normal process for the production of a ceramic shell mould involvesthe preparation of a pattern in wax or other material that isexpendable; building up round the pattern a shell of refractory materialby applying a number of coatings of a slurry made of powdered refractoryin a liquid binding agent (such as one derived from ethyl silicate),usually with intermediate stucco coatings of a somewhat coarserrefractory, and subjecting the dried assembly to a process such that thepattern is removed, for instance melted out. The shell is then fired.

The ceramic shell mould is preferably immersed in the active carbonimmediately after withdrawal of the mould from the firing furnace, andusually the metal is poured into the mould without delay.

The mould can be placed in a static bed of active carbon or particulatematerial including active carbon, but the use of a fluidized bed isusually preferred.

The invention is illustrated by the following examples.

EXAMPLE 1 A ceramic shell mould was produced by applying successivecoatings of a slurry of sillimanite of particle size less than 200B.S.S. mesh in hydrolyzed ethyl silicate solution and a stucco ofsillimanite of particles size -40+80 B.S.S. mesh, to an assembly of waxpatterns, each slurry coating being allowed to set before applying thenext. A total of six slurry coatings was applied, and after the finalcoating had set the assembly was dried in a stream of warm air untilexcess alcohol and water had been removed.

The wax assembly was then removed from the mould by treatment in a steamautoclave, after which the mould was prepared for casting in the usualmanner by firing in a furnace at 1050 C. for one hour.

The hot mould was withdrawn from the firing furnace and immediatelyplaced in a mild steel cylindrical flask and embedded in activatedcarbon to within approximately one centimetre of the rim of the pouringcup. The activated carbon was material produced by the pyrolysis ofcoconut shells and commercially available under the name Ultrasorb. Itsparticle size distribution was as follows:

BS. 481 sieve: Percent 30 rnesh+40 mesh 43 40 mesh+60 mesh 45 60 mesh+80mesh 80 mesh+l00 mesh 2 A few seconds later a molten steel having thefollowing composition:

was poured into the mould to make the castings, and allowed to cool tobelow red-heat before stripping from the mould.

The resultant castings were found to have an excellent smoothblemish-free surface following a light shot-blasting operation to removeremnants of mould material adhering after the knock-out operation.

4 EXAMPLE 2 Shell moulds were surrounded by activated carbon asdescribed in Example 1 during the casting and cooling of a ferriticstainless steel having the following composition:

Percent C 0.09 Mn 1.48 Si 0.23 Ni 1.31 Cr 12.25 Mo 0.61 V 0.34 FeBalance The castings had excellent surfaces completely free frompitting.

In a comparative test, the activated carbon was replaced by graphite ofthe following particles size distribution:

Percent +10 mesh 4.5 +16 mesh 40.0 +20 mesh 46.0 +30 mesh 86.0 +40 mesh95.0

The surfaces of the castings were marred by pitting characteristic ofcast ferritic stainless steel.

EXAMPLE 3 By surrounding shell moulds with activated carbon as describedin Example 1, castings having excellent surfaces were obtained fromsteels of the following compositions:

EXAMPLE 4 This example describes an experiment providing a comparisonbetween the use of active carbon to surround a ceramic shell mouldaccording to the invention, and the use of a charcoal for the samepurpose.

A wax assembly was made consisting of two one-inch square bars of waxeight inches long joined at one end to a common runner-bar to the centreof which a conventional wax pouring cup was attached. The one-inch barswere parallel and separated by a distance of four inches.

This design enabled the ceramic shell mould made therefrom to have thetwo legs embedded in separate containers before casting. In onecontainer Ultrasorb formed the embedding material and in the othercontainer charcoal having a comparable particle size was used.

Metal of the following composition was cast in the mould:

The bar of metal cast in the charcoal bed showed some pitting and aconsiderable number of blow-hole defects, whereas the bar east in thebed of Ultrasorb had a satisfactory surface.

